The present invention relates to a method of applying cyanoacrylates, and, more particularly, to such a method permitting cyanoacrylates to be applied by brushing. While the method has specific application to fingernail extension systems, it also provides for other applications such as the use of cyanoacrylates as coatings and adhesives.
The present invention is targeted primarily at retail packages for fingernail enhancement systems, specifically, fingernail strengthening systems and fingernail extension systems. An object of the present invention is to provide a synergistic combination of product, package and method of application for such systems, as is available for the most widely recognized fingernail enhancement systems, i.e. nail polishes. Fingernail polish is safe and effective for its intended purpose, it is readily applied by brushing, and is conveniently packaged in a small bottle with a brush incorporated with the bottle cap. What is needed is a comparably attractive combination of product, method and packaging for fingernail strengthening and extension systems.
Acrylics have provided a variety of materials pertinent to fingernail strengthening and extension. Some acrylic formulations can be conveniently applied to an existing nail by brushing. Brushable acrylics include anaerobic, reactive and aerobic acrylics, all of which polymerize by a free-radical mechanism.
Polymerization of anaerobic acrylics is normally prevented by the presence of oxygen. Anaerobic acrylics can be formed, for example, by blending a diacrylate or dimethacrylate based on diethylene glycol or higher homologs with a long half-lived hydroperoxide. The very small instantaneous concentration of radicals produced by the homogenous decomposition of the hydroperoxide combines with the diacrylate or alternative to initiate polymerization. However, propagation does not proceed normally in the presence of even small amounts of oxygen which reacts preferentially with the monomer radical to form inactive species.
Anaerobic characteristics can be imparted to several acrylate ester monomers by adding a peroxy polymerization initiator. Examples of suitable monoacrylate ester monomers are furfuryl methacrylate, cyclohexyl acrylate, isobutyl methacrylate, and hydroxyethyl methacrylate.
Even in the absence of oxygen, most anaerobics cure slowly in the absence of accelerators. With the exception of certain "active" metal surfaces, when ions from the surface act as accelerators for the rate of cure, accelerators are generally required for practical cure periods. Anaerobics also generally require surfaces that are clean, free from dirt or grease, and usually mechanically abraded.
From the point of view of fingernail extension procedures, anaerobic acrylics have proved workable but not entirely satisfactory. In the case of a procedure to be applied in a consumer's home, it is generally not practical to provide an oxygen-free cure environment. In addition, the necessity of surface preparation is undesirable.
In contrast to anaerobic acrylics, reactive acrylics depend on the presence of a chemical initiator, not the absence of oxygen, for their cure. Reactive acrylics generally comprise an elastomer colloidally dispersed in a monomer or in a monomer/oligomer/polymer solution. The resultant polymer is toughened by "elastomeric domains" resulting from the dispersed elastomer.
Reactive acrylics generally exhibit a high solvent action allowing the bonding of many unprepared surfaces, even oil contaminated surfaces. Thus, less surface preparation is required for bonding. In part, this is due to the presence of monomers in the reactive acrylic formulations that may dissolve contaminants and may also attack a polymer substrate.
The main disadvantage of reactive acrylics is that the user is required to combine two chemicals, the activator and the acrylic solution. This complicates both the packaging of a fingernail extension system and the application by the user. Furthermore, the strength of the resulting formation is highly dependent on the concentration of activator. This puts a burden on the end user to measure and mix precisely, and this burden may be unacceptable.
Additionally, reactive acrylics tend to be toxic. In some instances this is associated with methylmethacrylate and methacrylic acid monomers. These constituents of many reactive acrylics are toxic with respect to inhalation, ingestion and skin contact and have been rejected for reasons of toxicity on certain production lines. Typically low flash points and the resultant flammability hazard also are significant disadvantages for reactive acrylics. Furthermore, some of the more effective activators, such as dimethyl aniline, are suspected carcinogens.
Aerobic acrylics are similar to reactive acrylics in that they do not require the absence of oxygen or air to cure. Generally they require the use of pre-applied activators to initiate the cure mechanism. These materials are composed of catalysts, elastomeric domain fillers, and low vapor pressure monomers. While these tend to be less toxic than reactive acrylics, the need to combine chemicals remains objectionable.
Some aerobic acrylics are cured by exposure to long wave ultraviolet (UV) radiation. These materials are discussed by Andrew G. Bachman in "Ultraviolet Light Curing `Aerobic` Acrylic Adhesives", Adhesive Age, December 1982, pp. 31-35. The UV cure avoids the necessity of adding an activator. Furthermore, aerobics tend to be less toxic than the reactive acrylics. Thus, the aerobic acrylics have found a place in fingernail extension systems. However, UV lamps constitute a significant expense. Even when used correctly, UV lamps can be injurious. Furthermore, UV lamps are easily misused and can contribute to burns, eye damage and cancer. Also, the typical 3-5% shrinkage during UV cure can cause unsightly distortions.
Non-acrylic materials, such as formaldehyde resins and free formaldehyde have been used as fingernail strengtheners. However, these materials are carcinogens and impose unacceptable health risks on users.
High quality fingernail extensions and finger nail strengtheners can be formed from cyanoacrylates. Cyanoacrylate formulations have several characteristics which can be exploited by fingernail enhancement systems. Cyanoacrylates cure through a single component process, obviating the need for metering or mixing. Yet, accelerators are generally available where more rapid cure is desired. Even without accelerators, setting can occur in a matter of seconds or minutes, although complete cure can require a day or two. Cyanoacrylate formulations can be 100% reactive, minimizing the need for solvent evaporations and the concomitant health and environmental hazards imposed by such solvents.
Cyanoacrylates have a base of an alkyl 2-cyanoacrylate compound. The versatility of esters of alpha-cyanoacrylic acid, otherwise known as "cyanoacrylates", is well known. These alpha-cyanoacrylate esters may be represented by the general formula: ##STR1## wherein R can be any of various ester substitutions, including --CH.sub.3, --CH.sub.2 CH.sub.3, --CH.sub.2 CH.dbd.CH.sub.2, and --CH.sub.2 CH.sub.2 CH.sub.3. When applied in a monomeric form to a surface, they set up rapidly to give a high strength polymer structure, depending on application. The predominant esters are methyl and ethyl, but n-butyl and allyl are used occasionally.
For many years, it has been recognized that the cyanoacrylate esters are polymerized by the presence of --OH groups to form a strong bond in a short period of time without the necessity of a catalyst. Cyanoacrylates polymerize by an ionic mechanism initiated by moisture and basic ions as provided by ammonia or an amine or other organic base. Although the mechanism by which these cyanoacrylates cure is not completely understood, cyanoacrylates appear to polymerize rapidly when spread in a thin film because of facile attack by moisture adsorbed on the substrate surface.
Despite the advantages of cyanoacrylates, their usefulness has been limited by the difficulty in applying the formulations uniformly over a surface. Many people are familiar with the application of cyanoacrylates as glues, such as "Eastman 910" and "Crazy Glue." Such glues are distributed in tubes which can be opened and squeezed to eject the glue like toothpaste is usually squeezed onto a toothbrush. The simile conveys that this method does not yield uniform distribution. It is difficult to squeeze cyanoacrylates accurately and uniformly so as to avoid skin contact and to yield a smooth and level final formation. Thus, it is difficult to achieve an appropriate cosmetic look using this technique.
In the case of their use as glues, cyanoacrylates are often packaged with a plastic "spatula" or spreader, usually built into the cap for the tube. The ejected glue can then be pressed and shoved with this spreader to provide a more even distribution. However, those using this tool and technique have not generally achieved the uniformity desired for fingernail extensions. Other application approaches use squeeze bottles and droppers. These approach generally result in undesirable ridges rather than smooth uniform coats.
Spraying is not a viable approach to applying cyanoacrylates since their rapid cures easily clog spray heads. Also, there is a problem due to the irritation cyanoacrylates can cause to mucous membranes and eyes. While considered safe in contact with fingernails, contact with skin is to be avoided, and propelling the material through the air where it can reach eyes and mouths is dangerous.
A desirable alternative would be to brush cyanoacrylate formulations onto a selected surface. Unfortunately, the cure mechanisms for cyanoacrylates are rapidly activated on a brush, which is rendered useless in a short time. Generally, when one attempts to brush cyanoacrylates, the formulation coagulates and hardens before the formulation can be applied to a surface. While various means can be used to delay curing so that one or two brush strokes may be achieved, the cost involves destruction of a brush with each application.
Thus, what is needed is a synergistic system of product, method and packaging for fingernail extension and strengthening. The product should be relatively safe and effective for strengthening and extending fingernails. The method should be readily implemented and permit the uniform application of the product to a selected surface. The packaging should provide a shelf life appropriate to a consumer product and a convenient form for storing and applying the product.